High sensitivity solvent soluble developer of water-soluble crystalline material in a water-solvent vehicle

ABSTRACT

A METHOD FOR DETECTING FLAWS AND OTHER SURFACE DISCONTINUITIES BY THE LIQUED PENETRANT TECHNIQUE WHEREIN THE DEVELOPER COMPOSITION FOR THE DEVELOPMENT OF COLOR INDICATIONS IS A LIQUID VEHICLE COMNPRISING WATER AND A WATER-MISCIBLE ORGANIC SOLVENT, AND CONTAINS WATER-SOLUBLE CRYSTALLINE SUBSTANCES DISSOLVED THEREIN. UPON EVAPORATION OF THE LIQUED VEHICLE, THE SOLUTE CRYSTALLIZES OUT TO FORM A LAYER OF FINE CRYSTALS ON THE SURFACE UNDERGOING TEST. THE PENETRANT USED IN CONNECTION WITH MY METHOD PREFERABLY CONTAINS A VISIBLE OR FLOURESCENT DYE DISSOLVED THEREIN AND IS OF AN OILY-TYPE WHICH PREFERABLY IS NOT SELF-EMULSIFYING AND IS SUBSTANTIALLY FREE OF ANY ADDED EMULSIFYING AGENT BUT MY METHOD IS APPLICABLE WHERE THE PENETRANTS ARE WATER-WASHABLE. THE DEVELPOER COMPOSITION CAN OPTIONALLY INCLUDE SURFACTANTS AND FILM-FORMING INGREDIENTS, SEQUESTERING AGENTS FOR USE WITH HARD WATER, AND CORROSION INHIBITING AGENTS, BUT IN THE ABSENCE OF THESE OPTIONAL INGREDIENTS THE LIQUID DEVELOPER COMPOSITION UPON EVAPORATION OF THE LIQUID VEHICLE FORMS A PENETRANT PERMEABLE LAYER OF FINE CRYSTALS THAT CAUSE THE PENETRANT TO BE DRAWN BY CAPILLARITY OR OTHERWISE INTO THE CRYSTAL LAYER TO FORM FLAW INDICATIONS OF HIGH FIEDLITY AND RESOLUTION THAT CAN EASILY BE DETECTED UPON INSPECTION. THE REAMINING LAYER AFTER INSPECTIN CAN BE ADEQUATELY CLEANED OFF BY THE USE OF WATER ALONE. AN ADVANTAGE OF MY LIQUID DEVELOPER COMPOSITION IS THAT IT CAN BE SPRAYED ON TO THE SURFACE UNDERGOING TEST AND DUE TO ITS RELATIVELY GREATER VOLATILITY THAN WATER, IT EVAPORATES MORE QUICKLY.

United States Patent O 3,642,655 HIGH SENSITIVITY SOLVENT SOLUBLE DEVEL- OPER F WATER-SOLUBLE CRYSTALLINE MATERIAL IN A WATER-SOLVENT VEHICLE James S. Borucki, Chicago, Ill., assignor to Magnaflux Corporation, Chicago, Ill. No Drawing. Filed Feb. 11, 1969, Ser. No. 798,432 Int. Cl. G01n 31/22 US. Cl. 252-408 3 Claims ABSTRACT OF THE DISCLOSURE A method for detecting flaws and other surface discontinuities by the liquid penetrant technique wherein the developer composition for the development of color indications is a liquid vehicle comprising Water and a Water-miscible organic solvent, and. contains water-soluble crystalline substances dissolved therein. Upon evaporation of the liquid vehicle, the solute crystallizes out to form a layer of fine crystals on the surface undergoing test. The penetrant used in connection with my method preferably contains a visible or fluorescent dye dissolved therein and is of an oily-type which preferably is not self-emulsifying and is substantially free of any added emulsifying agent but my method is applicable where the penetrants are Water-washable. The developer composition can optionally include surfactants and film-forming ingredients, sequestering agents for use with hard water, and corrosion inhibiting agents, but in the absence of these optional ingredients the liquid developer composition upon evaporation of the liquid vehicle forms a penetrant permeable layer of fine crystals that cause the penetrant to be drawn by capillarity or otherwise into the crystal layer to form flaw indications of high fidelity and resolution that can easily be detected upon inspection. The remaining layer after inspection can be adequately cleaned off by the use of Water alone. An ad vantage of my liquid developer composition is that it can be sprayed on to the surface undergoing test and due to its relatively greater volatility than water, it evaporates more quickly.

CROSS-REFERENCE TO RELATED APPLICATION This invention relates to an improved liquid developer composition over that disclosed in the Borucki and Graham pending application Ser. No. 678,176, filed Oct. 26, 1967, now Pat. No. 3,561,262, issued Feb. 9, 1971, wherein the liquid vehicle of the developer composition consisted of water itself.

SUMMARY OF THE INVENTION In accordance with the present invention, the liquid for the developer composition is prepared by mixing Water with a water-miscible organic solvent within the broad limits of between about 30 to 70% by weight of the water-miscible solvent with from about 70 to 30% of Water. Water soluble substances are then dissolved in the resulting liquid vehicle Within the limits of about 5% up to saturation, and preferably from about to saturation, the latter being dependent upon the solubility of the particular water-soluble substance used.

All of the water-soluble crystallizable substances used in the liquid developer composition of my invention have solubilities in Water greater than about 5 parts per 100 parts of water by weight and have similar minimum solubilities in the liquid vehicles chosen within the above given ranges of water and organic solvent. Where greater solubility of the crystalline substance is desired, the liquid developer composition can be used at elevated temperatures up to 100 F., or even higher, but in general the 3,642,655 Patented Feb. 15, 1972 ice temperature at which the developer composition is used is approximately that of the circumambient air.

The crystalline substances useful for my developer composition may be either inorganic, organic or metalloorganic, provided they are water-soluble and soluble to the extent of at least 5%, and preferably 10% by weight, of the liquid vehicle. The following are some of the Watersoluble inorganic crystalline substances suitable for use: sodium nitrite, sodium bicarbonate, sodium carbonate, sodium nitrate, sodium chloride and the corresponding potassium compounds.

Suitable organic compounds are as follows: urea, sucrose, resorcinol and ethylene diamine tetramine acetic acid, sodium salt.

Suitable metallo-organic salts as follows: sodium benzoate, ammonium acetates, sodium gluconate, sodium citrate, sodium salicylate, and the corresponding potassium salts.

Of the foregoing crystallizable substances, sodium benzoate, and sodium nitrite are preferred. However, any of the foregoing crystalline substances can be used alone or in admixture with one or more of the other named crystalline substances to give a developing action, but with some sacrifices in performance as compared with the preferred substances.

Of the organic solvents that are suitable, isopropanol is the preferred solvent, but anhydrous ethanol, methanol, propanol and acetone are all suitable for use. The preferred blend of solvents is 50/50 water and isopropanol (anhydrous), but any of the other organic solvents can be substituted in place of the isopropanol, and the proportions can be varied between 30 and 70% of the organic Water miscible solvent, and 70 and 30% of water.

If desired, a surfactant can be used, such as any of those disclosed in the commonly o'wned Borucki and Graham application above referred to, and particularly pages 13 to 15 thereof.

Similarly, a film-forming water-soluble ingredient can be incorporated into my developer composition, such as Carbowax 4000, which is a proprietary synthetic, watersoluble wax having a water solubility of grams per 100 cc. at 20 C., and other water-soluble film-formers such as those set forth on page 16 of the same Borucki et a1. application.

One of the advantages of my present liquid developer composition, however, is that it does not require a surfactant or a water-soluble film-former. Consequently, the broad range of ingredients of the developer composition is as follows:

Soluble crystalline substance-5% to saturation or above Water dispersible surfactant-0 to 5% Water-soluble film-former--0 to 10% Liquid vehicle-Balance The liquid vehicle comprises from 30 to 70% of the organic water-miscible solvent, and from 70 to 30% of Water.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The preferred developer composition of my invention has the following formula, expressed in percentages by weight:

Formula No. 1: Percent by weight Sodium benzoate 28 Osopropyl alcohol 30 Water 42 Formula No. 2:

Sodium benzoate 14 Sodium nitrite 14 Isopropyl alcohol 36 Water 36 3 Formula No. 3: Percent by weight Sodium benzoate 15.5 Sodium nitrite 15.5 Surfactant 1 Isopropyl alcohol 34 Water 34 Formula No. 4:

Sodium benzoate *6 Surfactant 0.1 Carbowax 4000 5 50/50 Isopropyl alcohol/water Balance Formula No. 5:

Sodium benzoate 12.5 Urea 12.5 50/50 acetone/Water 75.0

In place of sodium benzoate or sodium nitrite, any of the named water-soluble crystalline substances, depending upon solubilities, can be used in the same proportions :as above indicated but perhaps with some sacrifice in performance. Likewise, any of the water-miscible organic solvents can be used in the above formulae in place of isopropyl alcohol in admixture with Water within the ranges of 30 to 70% of organic solvent to 70 to 30% of water.

In general, increases in the organic solvent content of the liquid vehicle to above 50% by weight do not appreciably improve the spraying or drying characteristics of the developer. On the other hand, less than about 30% of solvent, 70% Water, results in a wetter and slower drying developer film, and that is not desired in spray applications.

Where several Water-soluble crystalline substances are used, the following formula is particularly satisfactory:

Formula No. 6: Percent by Weight Sodium dichromate 0.025

Sodium benzoate 11.0 Sodium nitrite 11.0 Anhydrous isopropanol 30.0 Water Balance In the use of the liquid developer composition of my invention, the developer composition is applied after the application of the colored liquid penetrant and the removal of the excess penetrant from the surface of the workpiece without removal of penetrant from any flaws or other surface discontinuities. The excess of penetrant is removed without removal of the penetrant residue entrapped in any discontinuities, and the developer composition is thereafter applied to said surface.

The application of the liquid developer composition is preferably carried out at ambient temperatures by spraying with conventional spray guns under a relatively high pressure, such as 30 pounds per square inch or above, but excellent results are obtained at much lower pressures. During the developing stage it is not necessary to dry the workpiece in a hot air circulating dryer, since evaporation of the liquid vehicle will take place upon mere standing in the atmosphere.

Upon evaporation of the liquid vehicle, a myriad of light-colored, or white, fine crystals are left on the surface of the workpiece as a dry penetrant-permeable layer. Development occurs as soon as the crystals are dry. Penetrant is drawn up into the layer of crystals by capillarity to give indications of the defects that are enlarged by migration of the dyed liquid penetrant onto the surfaces of the crystals.

The use of my liquid developer composition is particularly economical as to cost and equipment where the penetrant method is used in a big automated system and large structural parts, such as parts of an airplane structure, are to be inspected. The crystalline water-soluble substances for which preference has been expressed herein impart a high degree of brightness to the color indication, whether the color of the penetrant is a visible or a fluorescent color. Inspection of the color indication is, of course, made under an appropriate light or exciting radiation, such as white light for the visible colors and black light for the fluorescent colors.

The liquid vehicle of the penetrant used is preferably of the oily type, such as a liquid hydrocarbon solvent, which is immiscible with water and which contains no added surfactant, wetting agent or emulsifying agent. However, a water-washable penetrant can be used. In the absence of any surfactant, the permeable layer of (fine crystals, deposited on the surface of the workpiece upon evaporation of the liquid vehicle, serves to draw up any entrapped liquid penetrant, aided by capillary and other forces, into said crystal layer to coat the crystals thereof with the liquid penetrant. After completion of the developing action and the inspection of the developed color indications, the residual crystal layer being completely water-soluble can readily be washed off with water, as by flushing, spraying or other means, to leave a clean surface.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim:

1. In a method of detecting surface discontinuities in which a liquid penetrant is applied to the surface of a test piece to become entrapped in any discontinuities or flaws therein, the excess of penetrant is removed without removal of entrapped liquid penetrant residue and a developer is thereafter applied to said surface to enhance the visibility of flaw indications including said entrapped liquid penetrant residue, the improvement which comprises:

applying as said developer a solution consisting essentially of from 30 to 70% by weight of water and from 70 to 30% of a water-miscible organic solvent as the liquid vehicle, and from 5% by weight to saturation of a water-soluble crystalline substance dissolved in said liquid vehicle, and

effecting the evaporation of said applied solution to cause the deposition therefrom on said surface of said crystalline substance as a completely watersoluble layer of crystals permeable to and coated by said liquid penetrant residue to give a fiaw indication of enhanced visibility.

2. The method as defined by claim 1, in which said liquid penetrant is an oily type penetrant said liquid vehicle contains a dye dissolved therein, and said liquid penetrant residue is drawn up into said water-soluble permeable layer by capillary force to coat said crystals.

3. The method as defined by claim 2, in which said liquid penetrant is a water immiscible petroleum hydrocarbon solvent free of any emulsifying agent and contains a fluorescent dye dissolved therein.

References Cited UNITED STATES PATENTS 2,667,070 1/1954 Sockman 252-408 X 2,707,236 9/ 1949 De Forest 250-71 2,764,556 9/1956 Sockman et al. 252-408 2,806,959 9/1957 De Forest et al. 73-104 X 3,418,078 12/1968 Mlot-Fijalkowski 252-408 X 3,506,827 4/1970 Alburger 25230l.2 X

JOHN T. GOOLKASIAN, Primary Examiner M. E. MCCAMI'SH, Assistant Examiner U.S. Cl. X.R. 

